Fix CVE-2021-3498:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3498
https://gstreamer.freedesktop.org/security/sa-2021-0003.html

Patch copied from upstream source repository:

https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/commit/02174790726dd20a5c73ce2002189bf240ad4fe0?merge_request_iid=903

diff --git a/gst/matroska/matroska-demux.c b/gst/matroska/matroska-demux.c
index 4d0234743b8cf243b4521e56ef9027ba23b1b5d0..467815986c8c3d86fd8906a0d539b34f67d6693e 100644
--- a/gst/matroska/matroska-demux.c
+++ b/gst/matroska/matroska-demux.c
@@ -692,6 +692,8 @@ gst_matroska_demux_parse_stream (GstMatroskaDemux * demux, GstEbmlRead * ebml,
 
   DEBUG_ELEMENT_START (demux, ebml, "TrackEntry");
 
+  *dest_context = NULL;
+
   /* start with the master */
   if ((ret = gst_ebml_read_master (ebml, &id)) != GST_FLOW_OK) {
     DEBUG_ELEMENT_STOP (demux, ebml, "TrackEntry", ret);
